For many years, large scale gas turbine based stationary power plants fueled by conventional gas or liquid fuels, such as natural gas, liquefied petroleum gas (LPG), etc., have been used by utilities for peaking purposes because of the fast, on-line response of a gas turbine. More recently, combined cycle power plants have been constructed to increase the efficiency of the system thus enabling the gas turbine to be incorporated into power plants that provide base load electric power. Such power plants include a waste heat boiler that extracts heat from the exhaust gases of the gas turbine for generating steam that drives a steam turbine and produces additional power. Furthermore, sometimes supplementary duct firing has been used as an addition to the waste heat boiler.
At the same time, efforts have been made to utilize less expensive, and more abundant low heat content solid fuels, such as low quality coal, oil shale, bituminous phosphates, biomass, etc., in base-line power plants. Conventionally, power plants using such low heat content fuels have employed fluidized bed combustors that include integral heat transfer elements. Such elements are usually rapidly fouled by agglomeration of ash because the solid fuels used contain relatively large amounts of ash, and this complicates heat transfer to the working fluid of the power plant.
It is an object of the present invention to provide a combined cycle power plant operating on low quality solid fuel in which several of the disadvantages outlined are reduced or substantially overcome.